Find a Theme
What is it that will set your event apart from all the others. For example, are you going for a speed record? Is your marathon centered around a competition? Or is it just a casual family letting you into their home for the weekend? There are only a handful of game series around which to base a marathon, so very soon it will be impossible to do one without repeating games that someone else has done. Having a unique theme, or something different will set your marathon apart from the ones that have come before and help bring in more viewers.